The invention relates to a device for detecting a hot point in a scene by means of infrared radiation. The device comprises a scanning system, a radiation-sensitive detection system for converting an infrared beam obtained into an electric signal, and an electronic processing circuit for processing the signal. The invention also relates to a missile guidance system equipped with such a device.
The infrared radiation sensed by the device has a wavelength greater than one micron. In order to obtain correct observations, allowance is to be made for the transparency of the atmosphere to the infrared wavelengths sensed. In practice two wavelength bands are employed, the first one being situated between 3.5 and 5 microns and the second one between 8 and 13 microns.
In order to detect a hot point, which has a temperature of for example 1500.degree. K., in a scene with an average temperature of 300.degree. K., the first band of wavelengths may be used. The hot point at 1500.degree. K. has a maximum luminance L.lambda.. However, this gives rise to excessive contrast between the luminance of the hot point and the luminance of the various other points of the scene. The hot point produces a response of the detection system which is spread in time and has such a level that it makes responses produced by other parts of the scene. Thus, the hot point will therefore be observed as a spot which obscures the details of the scene.
The scene may also be observed in the second wavelength band, in which the contrast is smaller. However, in that case, if the hot point is viewed at an angle smaller than that of the elementary fields in which the scene is divided by the scanning system, the apparent luminance decreases with the square of the distance, so that a hot point which is situated too far can no longer be distinguished.
As proposed in German Patent Application No. 2,533,270, it is also possible to employ two bands of wavelengths at the same time, the first band only being employed for detecting the hot point and the second band for observing the scene. This presents problems with the scanning system. It is difficult to design such a system so that it performs satisfactorily in both wavelength bands at the same time.